The normal skin color at most sites on the human body is generally pink. Skin color depends on the amount of blood flowing in the capillaries through which blood flows from the arterioles to the venules. The present invention resides in non-invasive invasive detection of hemodynamic changes in the skin arteriolar-capillary flow during states of pre-shock, shock and cardiopulmonary distress. These changes are indicative of a reduction in blood delivery to an organ of the body.
Expressed in its simplest terms, shock is the consequence of an inadequate delivery of blood to a major organ of the human body. Unless shock is promptly treated, this deprivation of blood may give rise to a disturbance in the metabolism of the organ with a resultant damage thereto. Because of the serious consequences of shock, it detection and treatment is regarded medically as an emergency procedure in which time is of the essence.
Cellular damage to an organ may be reversed by prompt treatment of shock. But it is otherwise irreversible and may lead to the death of the patient. Recovery from shock therefore depends on the promptness of treatment. However, before a patient can be treated for shock he must first be diagnosed to determine whether the patient is actually experiencing shock.
The treatment to be administered to a patient in shock depends on the nature of his condition. For example, for some shock conditions the appropriate treatment includes fluid resuscitation and the drug dopamine which acts to increase arterial perfusion pressure. Treatment for a shock condition must be administered with extreme care while the patient is being monitored.
A significant aspect of diagnostic instrumentation in accordance with the invention is that it is adapted to monitor as well as to detect shock-related conditions in a non-invasive manner. Using this instrumentation, one can make, even in a pre-hospital setting, an early diagnosis of shock as well as determine whether the drug being administered to a patient in shock is having the desired therapeutic effect.
Medical authorities classify shock syndrome in the following five categories:
(1) Hypovolemic shock
(2) Septic shock
(3) Cardiogenic shock
(4) Obstruction to cardiac filling shock
(5) Neurogenic shock
Hypovolemic shock, the most common type of shock, is caused by a massive loss of blood, plasma or fluid from the body of a patient, or the loss of fluid from an intravascular compartment. Such losses may be due to dehydration, vomiting, diarrhea, burns, or because of the abusive use of diuretics. A loss of blood and plasma is experienced in hemorrhagic shock such as in cases of blunt and penetrating trauma injuries, gastrointestinal bleeding, or Gynecologic/Obstetric bleeding. Many cases of bleeding are occult (e.g. slow internal bleeding), and therefore can not be diagnosed early.
Septic shock is caused by bacterial infection in which an endotoxin is released into the blood stream. The sequestration and pooling of blood in various vascular compartments reduces the availability of blood for the perfusion of other vital organs.
Cardiogenic shock is usually attributed to a massive myocardial infarction caused by extensive damage to the myocardium. This may be the result of arrhythmia in a patient suffering from heart disease. In this category of shock syndrome, the heart fails to pump properly, with a consequent reduction in arterial blood.
Obstruction to cardiac filling shock takes place when this filling activity is lessened or arrested by a massive pulmonary embolism, or by space-occupying lesions. Neurogenic shock results from a severe spinal cord injury, or from a massive intake of a depressant drug, causing a loss of vasometric tone.
The five categories of shock syndrome each represent other causes of cardio-pulmonary distress, or a shock-related condition. The term “shock-related condition”, as used hereinafter is intended to embrace all five categories.
The onset of a shock condition is characterized by the reduction in blood flow to skin tissue (decreased skin perfusion). This reduction in skin perfusion is the result of a profound vasoconstriction of the skin tissue arterioles, which leads to decreased capillary flow, and a resultant poor perfusion to the skin. In order to diagnose an early stage of shock, one must detect this early reduction in skin capillary flow. A useful clinical, bed-side test for poor skin perfusion is an estimation of Capillary Filling Time (CFI). When applying pressure onto a specific skin area, the capillaries below the depressed area collapse and blood is blanched therefrom, thereby causing the skin color in the depressed skin area to whiten. Upon abrupt release of this pressure, blood flows back into the capillaries and the original skin color is recovered.
CFT is defined as the time it takes for the original pink skin color to return aft it had been blanked. In clinical practice, prolongation of the CFT for more than 2 second is considered a state of shock resulting from poor skin perfusion. This well-known bed-side tests although subjective and inaccurate, is an important vital sign of a shock state. If an appropriate treatment has not been given early enough, the shock condition will continue to deteriorate, the arteriolar vasoconstriction will increase even further, as reflected by prolongation of the CFT, blood pressure will fall and the patient may die. However, an appropriate prompt treatment at the early stage of shock will decrease vasoconstriction and shorten the CFT.
Known non-invasive methods to diagnose shock do not evaluate perfusion. These methods rely on the following cardiovascular parameters:
Blood pressure. An indirect parameter of shock. The measurement of blood pressure identifies shock only in its late stages when blood pressure drops (uncompensated shock).
Heart rate. An indirect parameter of shock. The specificity of this measurement is low because heart rate is also increased by other common physiological conditions, such as anxiety and pain.
The advantage gained by measuring the rate of blood perfusion by way of CFT instrumentation is that it enables early detection of a shock syndrome (compensated shock, prior to the reduction of blood pressure) and indicates its severity. This makes possible prompt treatment of patients who can then survive a shock-related condition which may be fatal if untreated or if treated too late.
Disclosed in U.S. Pat. No. 3,698,382 is an apparatus for measuring veno filling time which applies intermittent and uniform pressure to the skin of a patient. This instrument which measures capillary flow changes secondary to the compression of a vein comprises a light source for illuminating a skin area and photoelectric monitoring means sensitive to the coloration of the skin area. The instrument measures the rate at which color returns to the skin area after pressure thereon is released. However, there are major differences between the '382 apparatus and apparatus in accordance with the invention in that the former measures capillary flow changes resulting from mechanical pressure applied to a nearby vein and these changes in flow do not reflect a state of shock.
When measuring CFT it is essential that pressure be applied only to capillary vessels while maintaining normal blood flow. In a preferred embodiment of an apparatus in accordance with the invention, a programmable mechanical unit applies an accurate measurable amount of pressure to the skin.
In order to diagnose the condition of shock, one must detect capillary flow changes resulting from the physiologic stress of shock. These changes in capillary flow are due to vasoconstriction and are not related to mechanical pressure applied to a near by vein. When measuring CFT, it is vital that pressure be applied only to the capillary vessels while maintaining normal venous flow. In contradistinction to the apparatus in the '382 patent an apparatus in accordance with the invention uses a programmable mechanical unit that applies accurate measurable pressure to the skin, which increases gradually, until a point of maximal skin whitening has been detected. This technique makes it possible to find the MINIMAL blanching pressure which results in maximal whitening. At minimal blanching pressure, blood is moved away from the capillaries while maintaining normal flow in the veins. This technique is the hallmark of measuring true systemic changes in capillary flow.
The '382 patent apparatus is subject to interference from external light sources and therefore requires an opaque housing for the monitoring apparatus. The apparatus does not measure skin temperature which has an independent effect on capillary flow. In addition, the medical arrangement required for maintaining uniform pressure in order to attain more accurate readings is cumbersome and costly.
They are also relatively complex and expensive and difficult to interpret clinically (laser Doppler devices for example). Time is of the essence in the diagnosis and treatment of shock, yet known types of skin capillary flow instrument are incapable of facilitating rapid diagnosis and element of shock. It is vital that skin capillary flow instruments have a high order of accuracy so that their readings indicate the severity of the shock or shock-related condition.
Studies published in the medical literature over the last two years demonstrate that skin temperature independently influences the skin capillary flow. One major limitation of prior skin capillary flow measurement devices is that they do not take into account skin temperature, and therefore do not correlate the measurement to skin temperature. This correlation enables real-time analysis of the state of shock. In contradistinction, a device in accordance with the invention measures skin temperature prior to each CFT measurement, so that every CFT measurement is correlated to the change in skin temperature.
Of general background interest is U.S. Pat. No. 4,494,550 which discloses apparatus for the non-invasive detection of venous and arterial blood flow drainage disorders which is designed for the detection of flow abnormalities in the large vessels of a limb. Also of background interest is U.S. Pat. No. 5,050,613 (1991) which discloses a vascular testing apparatus. This includes capillary blood flow sensors to measure the blood flow of a patient. This diagnostic tool acts to determine the overall vascular integrity of a patient, but is unable and does not diagnose shock or shock-related conditions.